1. Field of the Invention
This invention relates to an optical head device and an optical information recording/reproduction apparatus adapted to show excellent recording/reproduction characteristics by correcting the influence of birefringence of the protection layer of the optical recording medium on incident light or reflected light.
2. Description of Related Art
FIG. 15A of the accompanying drawings schematically illustrates a known popular optical head device. Incident light from a semiconductor laser 1, which is a light source, is collimated by a collimator lens 2 and enters a polarization beam splitter 3 as P-polarized light so as to be transmitted through it substantially by 100%. Then, it is transmitted through a ¼ wave plate 4 and converted into circularly polarized light from linearly polarized light before it is converged on a disk 7, which is an optical recording medium, by means of an objective lens 6. Light reflected by the disk 7 is transmitted through the objective lens 6 and the ¼ wave plate 4 in the opposite direction, where it is converted into linearly polarized light with a direction of polarization orthogonal relative to the direction of polarization of forward-moving light from circularly polarized light, before it enters the polarization beam splitter 3 as S-polarized light and is reflected substantially by 100%. Then, it is transmitted through a cylindrical lens 8 and a convex lens 9 and received by a photo-detector 10.
Meanwhile, while the protection layer of the optical recording medium is normally made of inexpensive polycarbonate, polycarbonate gives rise to birefringence. If the protection layer of the disk 7 of FIG. 15A, which is an optical recording medium, gives rise to birefringence, the diameter of the spot of converged light that is formed on the disk 7 is enlarged and the light receiving rate of the photo-detector 10 falls.
It should be noted here that the protection layer refers to the substrate of an optical recording medium when light is made to strike the recording surface of the optical recording medium by way of the substrate, whereas the protection layer refers to the cover layer of the optical recording medium when light is made to strike the recording surface of the optical recording medium by way of the cover layer in the following description.
If the protection layer of the disk 7 is free from birefringence, light reflected by the disk 7 is transmitted through the ¼ wave plate 4 to become S-polarized light relative to the polarization beam splitter 3. Therefore, it is reflected by the polarization beam splitter 3 substantially by 100% and received by the photo-detector 10. However, if the protection layer of the disk 7 gives rise to birefringence, light reflected by the disk 7 is transmitted through the ¼ wave plate 4 to normally become elliptically polarized light. In other words, the S-polarized light component is reduced relative to the polarization beam splitter 3 and a P-polarized light component is produced. Then, while the S-polarized light component is reflected by the polarization beam splitter 3 substantially by 100% and received by the photo-detector 10, the P-polarized light component is transmitted-through the polarization beam splitter 3 substantially by 100% and returned to the semiconductor laser 1. This is why the light receiving rate of the photo-detector 10 falls.
The birefringence of the protection layer of an optical recording medium involves intra-plane or horizontal birefringence and vertical or perpendicular birefringence as described in Non-Patent Document 1 (Yoshizawa, “An Analysis of Optical Anisotropy of PC Substrate for Magneto-Optical Disc”, KOGAKU, Oct., 1985, Vol. 15, No. 5, pp. 414-421). Now, let us define the relationship between the disk 7, which is an optical recording medium, and an XYZ coordinate system as shown in FIG. 15B. The X-axis, the Y-axis and the Z-axis respectively agree with a radial direction, a tangential direction and a normal direction of the disk 7. The protection layer of an optical recording medium normally shows biaxial refractive index anisotropy and the three principal axes thereof substantially agree with the X-axis, the Y-axis and the Z-axis respectively. If the corresponding three principal refractive indexes are nx, ny, nz and the intra-plane birefringence and the vertical birefringence are respectively Δn ∥ and Δn⊥, the intra-plane birefringence is defined by Δn ∥=|nx−ny | and the vertical birefringence is defined by Δn ⊥=|(nx+ny)/2−nz | respectively.
Both the intra-plane birefringence and vertical birefringence enlarge the diameter of the spot of converged light that is formed on an optical recording medium and reduce the light receiving rate of the photo-detector. However, they differ from each other in terms of how they affect light being transmitted through the protection layer of the optical recording medium. While the influence of intra-plane birefringence does not depend on the angle of incidence of light relative to the optical recording medium, that of vertical birefringence is dependent on the angle of incidence of light relative to the optical recording medium. More specifically, the influence of vertical birefringence is nil when the angle of incidence is 0° but the influence increases as a function of the angle of incidence. For this reason, vertical birefringence is generally more influential than intra-plane birefringence relative to the recording/reproduction characteristics. Both an increase in the diameter of the spot of converged light and fall in the light receiving rate due to vertical birefringence entail a fall in the resolution and an increase of the crosstalk of the reproduced signal.
FIG. 16 illustrates the computationally determined relationship between the vertical birefringence of the protection layer of an optical recording medium and the diameter of the spot of converged light on an assumption that the wavelength of light from the light source is 405 nm and the numerical aperture of the objective lens is 0.65 while the protection layer of the optical recording medium is 0.6 mm thick. It will be appreciated that the diameter of the spot of converged light rapidly increases as the vertical birefringence rises. The vertical birefringence of the protection layer of any optical recording medium is uniquely determined by the material of the layer. It is about 0.0007 when it is made of polycarbonate. Thus, the diameter of the spot of converged light is about 0.523 μm when the protection layer is free from vertical birefringence, whereas it is enlarged to about 0.540 μm when the protection layer involves vertical birefringence that is produced by polycarbonate.
An optical head device disclosed in Patent Document 1 (JP(A)-2000-268398) comprises a liquid crystal panel for correcting the influence of the intra-plane birefringence of the protection layer of an optical recording medium on incident light or reflected light. The liquid crystal panel has an optic axis running in a predetermined direction and is adapted to offset the phase difference produced by the intra-plane birefringence by giving a predetermined phase difference to transmitted light.
However, the effect of correcting the influence of intra-plane birefringence of the liquid crystal panel of the above cited Patent Document 1 is not sufficient because the liquid crystal molecules in the liquid crystal panel are oriented in the same direction, although the direction of the optic axis for correcting the influence of vertical birefringence on incident light or reflected light and the phase difference produced by vertical birefringence vary as a function of the position in the liquid crystal panel.